Semiconductor laser with lens and method of manufacturing the same

Abstract: A semiconductor laser includes a semiconductor laser chip stack having active layers formed substantially in the central portion thereof in the stacking direction and a pair of opposed reflecting surfaces formed at both ends of the active layers, and includes a convex lens of a light transmissible material fixedly attached to a partially light transmissible reflecting surface of the pair of reflecting surfaces and adapted for converging a laser beam emitted from the partially light transmissible reflecting surface.

Description: The present invention relates to a semiconductor laser and a method of manufacturing the same, and more particularly to a semiconductor laser which is easy to manufacture and a method of manufacturing the same. Conventionally, a technique has been proposed to improve the luminous efficiency of radiation of a semiconductor laser. Such a prior art technique is disclosed, for example, in Japanese Laid-Open Utility Model Publication No. 2-92961. FIG. 12 shows a sectional structure of such a prior art semiconductor laser. In FIG. 12, a semiconductor laser chip 80 is formed by stacking in sequence an n-GaAs substrate 82, an n-Al 0.35 Ga 0.65 As clad layer 83, an undoped GaAs active layer 84, a p-Al 0.35 Ga 0.65 As clad layer 85 and a p-GaAs contact layer 86. An n-side electrode 81 is fixedly attached to the n-GaAs substrate 82, and a p-side electrode 87 is fixedly attached to the p-GaAs contact layer 86. An insulating protector coat 91 is fixedly attached to a radiant end surface 92 of the semiconductor laser chip 80. The insulating protector coat 91 has a projecting convex lens 91a formed at a portion opposite to the end of the active layer 84. With this construction, a laser beam emitted from the end of the active layer 84 can be converged by the projecting convex lens 91a. Thus, the luminous efficiency of radiation can be substantially improved. In the above prior art, however, the active layer 84 is not located in the central portion of the semiconductor laser chip 80 in the stacking direction thereof but in the vicinity of the p-side electrode 87. As seen in FIG. 12, the height of the semiconductor laser chip 80 in the stacking direction includes a height de of 2 to 20 µm and another height ds of 100 to 200 µm. Therefore, it has been impossible for the projecting convex lens 91a formed by resin dropping to have its optical axis positioned at the end of the active layer. Furthermore, design for a converging lens and others in an external optical system has been complicated. EP-A-0 536 945 discloses a semiconductor laser device wherein plural quantum wells are disposed in a central region. The semiconductor laser device further comprises a cylindrical convex lens which is attached to one of the partially reflective surfaces and is adapted for converging a laser beam emitted from the active layers. Moreover, WO 86/02491 discloses a dual wavelength structure wherein two edge-emitting devices are bonded with p-regions adjacent. The bonding medium is a conductive material that forms a common electrode between the devices. Each device is separately addressable. Efficient coupling of emitted light into a single fiber is accomplished by restricting the vertical and horizontal separation of the light emitting stripes. In addition, US-A-5 086 431 discloses a semiconductor laser chip having one or more lasing junctions which is mounted on one side of a thin sheet of thermally conductive material opposite to a second semiconductor laser chip of the same construction mounted on the opposite side. It is an object of the present invention to provide a semiconductor laser in which a projecting convex lens is easily formed and design for an external optical system is simplified, and also to provide a method of manufacturing the same. According to a first aspect of the present invention, the above objects are achieved by a semiconductor laser comprising a first semiconductor laser chip having stacked in sequence, on a semiconductor substrate of a first conductivity type, a first semiconductor clad layer of the first conductivity type, a first active layer and a second semiconductor clad layer of a second conductivity type opposite to the first conductivity type, a second semiconductor laser chip having stacked in sequence, on a semiconductor substrate of the second conductivity type, a third semiconductor clad layer of the second conductivity type, a second active layer and a fourth semiconductor clad layer of the first conductivity type, wherein said first semiconductor laser chip and said second semiconductor laser chip are stacked such that the first and second semiconductor laser chips form a single semiconductor laser stack with the second semiconductor clad layer and the fourth semiconductor clad layer being inside, the two active layers being in the central portion, and the semiconductor substrates being outside thereof in the stacking direction of said semiconductor laser stack, a pair of opposed reflecting surfaces formed at both ends of said active layers with one of said pair of reflecting surfaces constituting a partially light transmissible reflecting surface, a convex lens of a light transmissible material fixedly attached to said partially light transmissible reflecting surface and adapted for converging the laser beams emitted from said active layers and through said partially light transmissible reflecting surface, such that the laser beams from the two active layers are emitted symmetrically with respect to the optical axis of the convex lens and whereas the height of the convex lens in the stacking direction of the semiconductor laser stack is equal to the thickness of said semiconductor laser stack in the stacking direction. As the semiconductor laser has the active layers serving as radiant sections located substantially in the central portion of the semiconductor laser chip stack in the stacking direction thereof, the convex lens of a light transmissible material for converging the laser beam can be easily fixed to the partially light transmissible reflecting surface formed at the end of the active layers. Further, design for the external optical system can be simplified. According to a second aspect of the present invention, the above objects are achieved by a method for manufacturing the semiconductor laser according to claim 1 comprising the steps of forming the first laser chip by stacking in sequence, on the semiconductor substrate of the first conductivity type, the first semiconductor clad layer of the first conductivity type, the first active layer and the second semiconductor clad layer of the second conductivity type; forming the second laser chip by stacking in sequence, on the semiconductor substrate of the second conductivity type, the third semiconductor clad layer of the second conductivity type, the second active layer and the fourth semiconductor clad layer of the first conductivity type; stacking the first and second semiconductor laser chips such that the first and second semiconductor laser chips form a single semiconductor laser stack with the second semiconductor clad layer and the fourth semiconductor clad layer being inside; the two active layers being in the central portion and the semiconductor substrates being outside thereof in the stacking direction of said semiconductor laser stack; and fixedly attaching the convex lens of a light transmissible material to the partially light transmissible reflecting surface of the pair of reflecting surfaces formed at both ends of each of the first and second active layers and opposite to each other, such that the laser beams from the two active layers are emitted symmetrically with respect to the optical axis of the convex lens and whereas the height of the convex lens in the stacking direction of the semiconductor laser stack is equal to the thickness of said semiconductor laser stack in the stacking direction. According to this manufacturing method, the first and second semiconductor laser chips are stacked, so that the semiconductor substrate of the first conductivity type constituting a large part of the first laser chip in the stacking direction thereof and the semiconductor substrate of the second conductivity type constituting a large part of the second laser chip in the stacking direction thereof are located outside of the first and active layers, respectively. Thus, the first and second active layers are located substantially in the central portion of the semiconductor laser chip stack in the stacking direction thereof. This assures easy fixture of the convex lens of a light transmissible material for converging laser beam from the first and second active layers through the partially light transmissible reflecting surface formed at the end of the first and second active layers, and also assures simplified design for the external optical system. The above and other objects and aspects of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a first embodiment of the present invention; FIG. 2 is a sectional view illustrating a part of a manufacturing process of a second embodiment of the present invention; FIG. 3 is a sectional view illustrating another part of the manufacturing process of the second embodiment; FIG. 4 is a sectional view illustrating the manufacturing process subsequent to FIGS. 2 and 3; FIG. 5 is a sectional view of a practical example of the semiconductor laser chip of the second embodiment; FIG. 6 is a front view illustrating a part of the manufacturing process of the second embodiment; FIG. 7 is a plan view illustrating a part of the manufacturing process subsequent to FIG. 6; FIG. 8 is a front view illustrating a part of the manufacturing process subsequent to FIG. 7; FIG. 9 is a plan view illustrating a part of the manufacturing process subsequent to FIG. 8; FIG. 10 is a side view of FIG. 9; FIG. 11 is a plan view illustrating a part of the manufacturing process subsequent to FIG. 9; and FIG. 12 is a sectional view of a prior art semiconductor laser. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, first and second embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a sectional structure of a semiconductor laser according to the first embodiment. In FIG. 1, a first laser chip 10 is formed by stacking in sequence a first p + -GaAs substrate 11, a first p-Al x Ga 1-x As clad layer 12, a first undoped GaAs active layer 13, a second n-Al x Ga 1-x As clad layer 14, and a first n + -GaAs contact layer 15. A second laser chip 20 is formed by stacking in sequence a second n + -GaAs substrate 21, a third n-Al x Ga 1-x As clad layer 22, a second undoped GaAs active layer 23, a second p-Al x Ga 1-x As clad layer 24, and a second p + -GaAs contact layer 25. A solder layer 31 is used to join the surface of the first n + -GaAs contact layer 15 of the first laser chip 10 and the surface of the second p + -GaAs contact layer 25 of the second laser chip 20 to form a semiconductor laser chip stack 1. A p-side electrode 32 is fixedly attached to the surface of the first p + -GaAs substrate 11, and an n-side electrode 33 is fixedly attached to the surface of the second n + -GaAs substrate 21. A pair of first and second reflecting surfaces 34 and 35 opposite to each other are formed at both ends of each of the first undoped GaAs active layer 13 and the second undoped GaAs active layer 23. The first reflecting surface 34 is partially light transmissible, while the second reflecting surface 35 is formed of a highly reflective thin film having a substantially high reflectance in comparison with the first reflecting surface 34. A convex lens 36 made of a transparent epoxy resin which is a light transmissible material is fixedly attached to the first reflecting surface 34. The convex lens 36 may be formed by dropping a transparent epoxy resin on the first reflecting surface 34, immersing the first reflecting surface 34 in a transparent epoxy resin, or using a mold. The height of the convex lens 36 is substantially equal to the height of the laser chip stack 1, and the optical axis of the convex lens 36 is positioned around the ends of the active layers 13 and 23. An arrow 37 shows the direction of emission of the laser beam. The semiconductor laser chip stack 1 with the convex lens 36 of the above construction is obtainable by cutting a large preformed stack bar into a plurality of individual semiconductor laser stacks. In this case, the cutting plane is parallel to the plane in FIG. 1. Therefore, the convex lens 36 is not semi-spherical but semi-cylindrical in configuration. With this construction, the second clad layer 14 of the first laser chip 10 and the fourth clad layer 24 of the second laser chip 20 are joined through the first contact layer 15, the solder layer 31 and the second contact layer 25, so that the first substrate 11 constituting a large part of the first laser chip 10 in the stacking direction thereof and the second substrate 21 constituting a large part of the second laser chip 20 in the stacking direction thereof are located outside of the first active layer 13 and the second active layer 23, respectively. Thus, the first active layer 13 and the second active layer 23 are located substantially in the central portion of the semiconductor laser chip stack 1 in the stacking direction thereof. This permits easy formation of the convex lens 36 of a light transmissible material for converging the laser beams from the first active layer 13 and the second active layer 23. Furthermore, as the first active layer 13 and the second active layer 23 which emit laser beams are disposed adjacent to and symmetrically with respect to the optical axis of the convex lens 36, the laser beam obtainable has a radiating pattern symmetrical with respect to the optical axis of the convex lens 36. Thus, in case provision of any additional lens is required, the design of the external optical system becomes simplified. In the above embodiment, the shape of the convex lens 36 is not limited to the semi-cylindrical configuration but it may be others such as a semi-spherical configuration. FIGS. 2 to 11 show the manufacturing process of the semiconductor laser according to a second embodiment of the invention. In FIG. 2, a semiconductor laser chip 40 is formed by stacking in sequence through epitaxial growth an n-GaAs substrate 41, an n-GaAs buffer layer 42, an n-Al 0.4 Ga 0.6 As clad layer 43, an n-Al 0.3 Ga 0.7 As clad layer 44, a GaAs active layer 45, a p-Al 0.5 Ga 0.5 As clad layer 46 and a p + -GaAs contact layer 47. Further, an n-side electrode 48 constituted by AuGe/Ni/Au layer is fixedly attached to the lower surface of the n-GaAs substrate 41 as seen in the drawing, and a p-side electrode 49 constituted by Cr/Pt/Au layer is fixedly attached to the upper surface of the p + -GaAs contact layer 47 as seen in the drawing. In FIG. 3, a semiconductor laser chip 50 is formed by stacking in sequence through epitaxial growth a p-GaAs substrate 51, a p + -GaAs buffer layer 52, a p-Al 0.5 Ga 0.5 As clad layer 53, a GaAs active layer 54, an n-Al 0.3 Ga 0.7 As clad layer 55, an n-Al 0.4 Ga 0.6 As clad layer 56 and an n-GaAs contact layer 57. Further, a p-side electrode 58 constituted by Cr/Pt/Au layer is fixedly attached to the lower surface of the p-GaAs substrate 51 as seen in the drawing, and an n-side electrode 59 constituted by AuGe/Ni/Au layer is fixedly attached to the upper surface of the n-GaAs contact layer 57 as seen in the drawing. In FIG. 4, the p-side electrode 49 of the semiconductor laser chip 40 and the n-side electrode 59 of the semiconductor laser chip 50 are joined by a solder layer 60. FIG. 5 shows a practical structure of the semiconductor laser chip 40. The n-GaAs substrate 41 is 100 µm in thickness, the n-Al 0.4 Ga 0.6 As clad layer 43 1.5µm, the n-Al 0.3 Ga 0.7 As clad layer 44 0.5 µm, the GaAs active layer 45 0.05 µm, the p-Al 0.5 Ga 0.5 As clad layer 46 1.5 µm, and the p + -GaAs contact layer 47 1 µm. An n-side electrode 48a constituted by AuGe/Ni/Au layer is fixedly attached to the lower surface of the n-GaAs substrate 41 as seen in the drawing, and a p-side electrode 49a constituted by Cr/Pt/Au layer is fixedly attached to the upper surface of the p + -GaAs contact layer 47 as seen in the drawing. A current blocking SiN insulator layer 49b having a thickness of 0.5 µm is formed. The semiconductor laser 40a has a length L of 400 to 600 µm in the transverse direction as seen in the drawing. Owing to the current blocking SiN insulator layer 49b, the practical radiant area is limited within a portion of the GaAs active layer 45 enclosed by a dotted line 45a. In this case, the resonator length is set in the stacking direction to the plane in FIG. 5 and the laser beam is emitted in the direction. FIG. 6 is a front view of a laser bar 61 for producing a plurality of (eight in FIG. 6) semiconductor laser chips. The laser bar 61 provides a semiconductor laser chips formed by epitaxial growth on an n-type substrate and provided with depressed electrode portions 61a to 61h between adjacent raised portions defined by a current blocking insulator layer. FIG. 7 is a plan view of the laser bar 61. Both side surfaces of the laser bar 61 are covered with end surface coats 64a and 64b. FIG. 8 shows the laser bars 61 and 62 joined by a solder layer 63. The laser bar 62 has electrodes 62a to 62h which correspond to the depressed electrode portions 61a to 61h of the laser bar 61, respectively. The laser bar 62 provides semiconductor laser chips formed on a p-type substrate. Gaps defined between the electrodes 61a to 61h and the other electrodes 62a to 62h are filled with solder. FIG. 9 is a plan view of the laser bar combination in FIG. 8. In FIG. 9, a convex lens 65 is fixedly attached to the outside of the end surface coat 64a. FIG. 10 is a side view of the laser bar combination in FIG. 9. FIG. 11 shows cutting planes 66a to 66g for obtaining individual semiconductor laser chip stacks. This method is effective to produce a semiconductor laser like the one shown in FIG. 1. As described above, the semiconductor laser of the present invention is effective to facilitate formation of a convex lens fixedly attached to the semiconductor laser chip stack and design for an external optical system. Thus, the cost of the optical system including the semiconductor laser can be reduced. While the invention has been described with reference to preferred embodiments thereof, it is to be understood that modifications or variations may be easily made without departing from the scope of the present invention which is defined by the appended claims.

Claims: A semiconductor laser comprising: a first semiconductor laser chip (10, 50) having stacked in sequence, on a semiconductor substrate (11, 51) of a first conductivity type, a first semiconductor clad layer (12, 53) of the first conductivity type, a first active layer (13, 54) and a second semiconductor clad layer (14, 55, 56) of a second conductivity type opposite to the first conductivity type, a second semiconductor laser chip (20, 40) having stacked in sequence, on a semiconductor substrate (21, 41) of the second conductivity type, a third semiconductor clad layer (22, 43) of the second conductivity type, a second active layer (23, 44) and a fourth semiconductor clad layer (24, 45, 46) of the first conductivity type, wherein said first semiconductor laser chip (10, 50) and said second semiconductor laser chip (20, 40) are stacked such that the first and second semiconductor laser chips (10, 50, 20, 40) form a single semiconductor laser stack with the second semiconductor clad layer (14, 55, 56) and the fourth semiconductor clad layer (24, 45, 46) being inside, the two active layers (13, 54, 23, 44) being in the central portion, and the semiconductor substrates (11, 51, 21, 41) being outside thereof in the stacking direction of said semiconductor laser stack, a pair of opposed reflecting surfaces formed at both ends of said active layers with one of said pair of reflecting surfaces constituting a partially light transmissible reflecting surface (34), a convex lens (36) of a light transmissible material fixedly attached to said partially light transmissible reflecting surface (34) and adapted for converging the laser beams emitted from said active layers (13, 54, 23, 44) and through said partially light transmissible reflecting surface (34), such that the laser beams from the two active layers (13, 54, 23, 44) are emitted symmetrically with respect to the optical axis of the convex lens (36) and whereas the height of the convex lens in the stacking direction of the semiconductor laser stack is equal to the thickness of said semiconductor laser stack in the stacking direction. A method for manufacturing the semiconductor laser according to claim 1 comprising the steps of: forming the first laser chip (10, 50) by stacking in sequence, on the semiconductor substrate (11, 51) of the first conductivity type, the first semiconductor clad layer (12, 53) of the first conductivity type, the first active layer (13, 54) and the second semiconductor clad layer (14, 55, 56) of the second conductivity type; forming the second laser chip (20, 40) by stacking in sequence, on the semiconductor substrate (21, 41) of the second conductivity type, the third semiconductor clad layer (22, 43) of the second conductivity type, the second active layer (23, 44) and the fourth semiconductor clad layer (24, 45, 46) of the first conductivity type; stacking the first and second semiconductor laser chips (10, 20, 50, 40) such that the first and second semiconductor laser chips form a single semiconductor laser stack with the second semiconductor clad layer (14, 55, 56) and the fourth semiconductor clad layer (24, 45, 46) being inside; the two active layers (13, 54, 23, 44) being in the central portion and the semiconductor substrates (11, 51, 21, 41) being outside thereof in the stacking direction of said semiconductor laser stack; and fixedly attaching the convex lens (36) of a light transmissible material to the partially light transmissible reflecting surface (34) of the pair of reflecting surfaces formed at both ends of each of the first and second active layers (13, 54, 23, 44) and opposite to each other, such that the laser beams from the two active layers (13, 54, 23, 44) are emitted symmetrically with respect to the optical axis of the convex lens (36) and whereas the height of the convex lens in the stacking direction of the semiconductor laser stack is equal to the thickness of said semiconductor laser stack in the stacking direction.

Inventors: NAKANO JIRO, OHNISHI TOYOKAZU, SEKI AKINORI, NAKANO, JIRO, OHNISHI, TOYOKAZU, SEKI, AKINORI
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1994
Main classification: H01S3
IPC codes: H01S3, H01S5